Differential with passive thermal-management system

ABSTRACT

A differential assembly includes a housing defining an oil sump, a differential disposed in the housing, and a thermal-management system. The thermal-management system includes an oil pump in fluid communication with the sump, an oil-to-air heat exchanger external to the housing, and a passive valve assembly. The passive valve assembly has an inlet connected to the pump, a first outlet connected to the heat exchanger, and a second outlet connected to a conduit disposed within the housing. The valve assembly further has a valve movable to a first position in which the inlet and the first outlet are in fluid communication and to a second position in which the inlet and the second outlet are in fluid communication. The valve is configured to be in the first position in response to the oil exceeding a first threshold temperature.

TECHNICAL FIELD

This application relates to differential assemblies and morespecifically to differential assemblies having thermal-managementsystems.

BACKGROUND

Motor vehicles may include a differential on the drive axle to transmittorque produced by an engine to driven wheels of the vehicle. Thedifferential allows the driven wheels to rotate at different speedsrelative to each other. This allows the outer wheel to rotate fasterthan the inner wheel when the vehicle is turning.

A typical open differential includes a ring gear meshing with a piniongear that is fixed to a driveshaft. A differential carrier is fixed tothe ring gear and is supported for rotation within a housing of thedifferential. The carrier supports a pair of side gears and a pair ofspider gears in meshing engagement with the side gears. The side gearsare driveably connected to the driven wheels. The spider gears transmittorque from the carrier to the side gears to propel the vehicle. Opendifferentials have difficulty propelling the vehicle when one of thedriven wheels is on a low-traction surface as torque from the engine isrouted to the low-traction wheel resulting is wheel spin.

Limited-slip differentials were developed to overcome the drawbacks ofopen differentials. Typical limited-slip differentials include a clutchpack and a spring that cooperate to engage a side gear, associated withthe higher-traction wheel, with the carrier to provide engine torque toboth driven wheels.

SUMMARY

According to one embodiment, a differential assembly includes a housingdefining an oil sump, a differential disposed in the housing, and athermal-management system. The thermal-management system includes an oilpump in fluid communication with the sump, an oil-to-air heat exchangerexternal to the housing, and a passive valve assembly. The passive valveassembly has an inlet connected to the pump, a first outlet connected tothe heat exchanger, and a second outlet connected to a conduit disposedwithin the housing. The valve assembly further has a valve movable to afirst position in which the inlet and the first outlet are in fluidcommunication and to a second position in which the inlet and the secondoutlet are in fluid communication. The valve is configured to be in thefirst position in response to the oil exceeding a first thresholdtemperature.

According to another embodiment, a differential assembly include ahousing defining an oil sump, a differential disposed in the housing,and a thermal-management system. The system includes an oil pump influid communication with the sump, a spool valve having an inletconnected to the pump, a first outlet, a second outlet, and a spoolslidable to a first position in which the inlet is in fluidcommunication with the first outlet and to a second position in whichthe inlet is in fluid communication with the second outlet. The spoolvalve further has a chamber containing wax configured to move the spoolaccording to a temperature of the wax such that the spool is in thefirst position when the temperature of the wax is within a firsttemperature range and is in the second position when the temperature ofthe wax is within a second temperature range.

According to yet another embodiment, a viscous-dissipation heaterassembly includes a body including an inlet and an outlet, and a valvehaving a metering portion disposed in the body between the inlet and theoutlet and a driven portion external to the body. The valve isactuatable to adjust size of an opening defined between the meteringportion and the body. An actuator arrangement configured to actuate thevalve. The actuator arrangement includes a hydraulic cylinder defining ahydraulic chamber and an orifice opening into the chamber and having apiston disposed in the hydraulic chamber. The piston is biased in afirst direction and is configured to move in a second direction inresponse to fluid pressure within the chamber overcoming the bias. Adrive mechanism is connected between the piston and the driven portion.Movement of the piston in the first direction reduces the size of theopening and movement of the piston in the second direction increases thesize of the opening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a vehicle including a differentialassembly that includes a thermal management system.

FIG. 2 is an exploded view of the differential of FIG. 1.

FIG. 3 is a schematic diagram of the thermal management system.

FIG. 4 is a perspective view of the differential assembly according toone or more embodiments.

FIG. 5A is diagrammatical view of spool valve of the thermal managementsystem in a first position.

FIG. 5B shows the spool valve in a second position.

FIG. 5C shows the spool valve in a third position.

FIG. 6 is a cross-sectional view of a viscous-dissipation heater havinga fixed orifice.

FIG. 7 is a diagrammatical view, in partial cross section, of aviscous-dissipation heater assembly having a variable orifice.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention. As those of ordinary skill in the art will understand,various features illustrated and described with reference to any one ofthe figures can be combined with features illustrated in one or moreother figures to produce embodiments that are not explicitly illustratedor described. The combinations of features illustrated providerepresentative embodiments for typical applications. Variouscombinations and modifications of the features consistent with theteachings of this disclosure, however, could be desired for particularapplications or implementations.

Referring to FIG. 1, a vehicle 20 includes front wheels 22 and rearwheels 24. In the illustrated embodiment, the vehicle 20 is rear-wheeldrive and the rear wheels 24 are powered by a powertrain including anengine 32 and/or an electric motor. A transmission 34 is connected tothe engine 32. A driveshaft 36 may transmit power from the transmission34 to the rear wheels 24 via a differential assembly 26 (which issometimes referred to simply as a “differential”) and left and righthalf shafts 28, 30. The vehicle 20 could also be front-wheel drive,all-wheel drive, or four-wheel-drive, in which case, the front wheelsmay include an associated differential. In a front-wheel-driveconfiguration, the differential may be housed within the transmission.

Referring to FIG. 2, the differential assembly 26 includes a housing(not shown) and a differential 40 disposed in the housing. Thedifferential 40 may be an open differential, a limited-slipdifferential, a locking differential, or combinations thereof. Thedifferential 40 enables the left and right driven wheels 24 to havedifferent speeds during cornering. The differential 40 includes acarrier 50 supported for rotation within the housing and configured toreceive power from the powertrain. The carrier 50 may include an end cap(not shown). For example, the carrier 50 may support to a bevel gear(not shown) that meshes with a pinion gear fixed to the driveshaft 36.

The carrier 50 may support a pair of opposing first and second sidegears 52, 54 and a pair of opposing first and second spider gears 56, 58in meshing engagement with the side gears. A shaft 60 may extend throughthe carrier 50 to interconnect the spider gears 56, 58. The first sidegear 52 is configured to transmit torque to the left half shaft 30, andthe second side gear 54 is configured to transmit torque to the righthalf shaft 28. The half shafts may be splined to the side gears. Theside gears 52, 54 and the spider gears 56, 58 may be supported forrotation on the carrier 50.

Referring to FIGS. 3 and 4, a main source of losses within thedifferential assembly 26 is between the rotating components and the oil68. The viscosity of the oil 68 is dependent upon temperature. Theviscosity decreases with temperature. Therefore, differential losses canbe reduced by actively heating the oil to the operating range. Thelosses can also be reduced by using less viscous oils. These oils,however, sometimes have a lower maximum temperature and may requireactive cooling. To increase fuel economy and promote more uniformoperation, the differential assembly 26 includes a thermal-managementsystem 60 configured to thermally regulate the temperature of oil 68.The thermal-management system 60 may be mechanically powered andpassively operated, i.e., no electronics are required, or alternatively,may include one or more electric components. Used herein, “passive”refers to components that self-actuate without the need for electronics.The thermal-management system 60 may be configured to heat and/or coolthe differential oil 68 depending upon the embodiment employed.

The thermal-management system 60 includes a pump 62 having a pickup tube64 in fluid communication with an oil sump 66. The pickup tube mayinclude a magnetic strainer. The pump 62 may be a positive-displacementpump that is powered by either the powertrain or by an electric motor.The pump 62 may be disposed within a housing 70 of the differentialassembly 26 or may be at least partially external to the housing. Asillustrated in FIG. 4, the pump 62 may be mounted on a cover 72 of thehousing 70. The pump 62 may be mechanically driven by rotation of thecarrier 50. The pump 62 may include a portion that extends into thehousing through the front cover 72 and driveably connects to the carrier50. For example, the carrier 50 may include a second bevel gear thatmeshes with a pinion gear of the pump 62. In other embodiment, the pump62 may be coupled to the pinion gear of the driveshaft or driven by anyother rotating component of the differential 40.

Driving the pump 62 with the driveline allows the pump to be driven withvehicle inertia energy (free energy) when the vehicle is coasting orslowing down. A clutch or similar device (optional) may be used todecouple the pump 62 from the driveline when the vehicle is acceleratingso that the pump 62 is only driven with the free energy. If the pump 62is electric, a controller may be programmed to operate the pump in anefficient manner.

The pump 62 is in fluid communication with a valve assembly 74 via asupply conduit 76. The valve assembly 74 may be supported on an insidesurface 78 of the cover 72. The valve assembly 74 may be at leastpartial immersed in the oil 68, e.g., located within the sump 66. Apressure-release valve 80 may be disposed between the pump 62 and thevalve assembly 74. The pressure-release valve 80 may be packaged with inthe pump 62 or as a separate component. The valve assembly 74 switchesthe thermal-management system 60 between the various modes of operationsuch as heating mode, cooling mode, and bypass mode. The valve assembly74 may be passive and automatically actuate based on changes intemperature without the need for electronic control. For example, thevalve assembly 74 may include a material, e.g., wax, that thermallyexpands to actuate the valve assembly 74 based on temperature of the oil68. In alternative embodiments, the valve assembly may be electronicallycontrolled.

The valve assembly 74 includes an inlet 82 and at least two outlets. Thenumber of outlets may depend on the number of modes of thethermal-management system 60. In the illustrated embodiment, the valveassembly 74 includes three outlets 84, 86, 88 each corresponding to oneof the three modes (heating, bypass, and cooling). Otherthermal-management systems may only include two modes, such as heatingand bypass, cooling and bypass, or heating and cooling, in which case,the valve may include two outlets. The valve assembly 74 includes aninternal valve (not shown) that selectively connects the inlet 82 to oneof the outlets 84, 86, 88 to switch the thermal-management system 60between the modes of operation.

The thermal-management system 60 includes a heating loop 90 connected tothe outlet 84. The heating loop 90 is configured to increase thetemperature of the oil and provide the heated oil onto variouscomponents of the differential 40. The heating loop 90 includes aviscous-dissipation heater 92 such as nozzle or orifice plate that areconfigured to heat the oil. The viscous-dissipation heater 92 heats theoil by manipulating fluid pressure rather than using an electric heateror an external heat source. The viscous-dissipation heater 92 has anorifice that is smaller than the upstream conduit to create heat throughviscous dissipation (sometimes called viscous heating). The orificesubstantially boosts the oil pressure within the heating loop 90 so thatviscous dissipation generates enough thermal energy to heat the oil 68.The oil pressure within the heating loop 90 may be as high as 1000pounds per square inch (psi) depending upon a variety of factors such asflow rate of the oil, pipe size, oil type, orifice diameter,differential heating requirements, and the like. The heating loop 90 mayinclude different conduit and other component than the other loops toaccount for these higher oil pressures (the other loops may include oilpressures of less than 15 psi for example). The viscous-dissipationheater 92 may include an input side connected to the outlet 84 byconduit 94 and an output side. The viscous-dissipation heater 92 may bemounted on the inside surface 78 of the cover 72 or other portion of thehousing 70. The viscous-dissipation heater 92 sprays the heated oilwithin the housing 70. In other embodiments, an electric heater may beused instead of the viscous-dissipation heater 92.

The thermal-management system 60 may also include a bypass loop 100connected to the outlet 86. The bypass loop 100 recirculates the oil 68passing through the valve assembly 74 back to the sump 66. The bypassloop 100 may include one or more conduit 102 that circulates the oilfrom the valve assembly 74 back to the sump 66. The conduit 102 mayextend all the way back to the sump or may include a non-restrictivenozzle or other feature that sprays the oil within the differentialhousing 70.

The thermal-management system 60 may further include a cooling loop 104connected to the outlet 88. The cooling loop 104 may include a heatexchanger 106 that is mounted external to the differential assembly 26.For example, the heat exchanger 106 may be attached to a body or framecomponent 107 of the vehicle 20 (see FIG. 1). The heat exchanger 106 maybe an oil-to-air heat exchanger configured to transfer thermal energy ofthe oil 68 to the ambient air. The heat exchanger 106 may be placed in alocation that receives a substantial amount of ram air to improveefficiency. Ducts, scoops, or the like may be used to channel airflowthrough the heat exchanger 106. In some embodiment, a fan may beprovided to increase airflow. The cooling loop 104 may include a supplyconduit 108 connecting the outlet 88 to the heat exchanger 106 and areturn conduit 110 that circulates cooled oil from the heat exchanger106 back to the sump 66. Both the return and the supply conduit 110, 108and a may include multiple segments some of which are disposed insidethe housing 70 and some of which are external to the housing 70. Theconduit may have flexible portions to allow relative movement betweenthe differential assembly 26 and heat exchanger 106.

FIGS. 5A, 5B, and 5C illustrate one example embodiment of the valveassembly 74. Here, the valve assembly 74 is a spool valve that isactuated by a material having a relatively high coefficient of thermalexpansion, such as wax. The valve assembly 74 includes a housing 120defining a valve bore 122, which may be cylindrical in shape. A spoolvalve 124 is disposed within the valve bore 122 and includes a pluralityof lands 126, 128, and 130. The lands may be disk-shaped having adiameter that approximates the diameter of the bore 122. The lands areaxially spaced apart from each other on the spool valve 124 to createpassageways between adjacent ones of the lands. The body further definesan inlet 134 and three outlets 136, 138, and 140 in the illustratedembodiment. The inlet 134 receives pressurized oil from the pump 62. Theoutlet 136 is connected to the heating loop 90, the outlet 138 isconnected to the bypass loop 100, and the outlet 140 is connected to thecooling loop 104. The spool valve 124 is slidable along the bore 122 toselectively connect the inlet 134 in fluid communication with one of theoutlets. For example, the spool valve 124 includes a first position(FIG. 5A) in which the inlet 134 is in fluid communication with theoutlet 136 to place the thermal-management system 62 in heating mode, asecond position (FIG. 5B) in which the inlet 134 is in fluidcommunication with the outlet 138 to place the thermal-management system62 in bypass mode, and a third position (FIG. 5C) in which the inlet 134is in fluid communication with the outlet 140 to place thethermal-management system 62 in cooling mode. The spool valve 124 may bebiased to one of the positions, e.g., the first position, by a resilientmember 142 such as a coil spring or the like. The resilient member 142may be disposed within the bore 122 and connected between one of thelands, e.g., land 130, and the housing 120. An actuator 144 slides thespool valve 124 to switch between the first, second, and thirdpositions.

The actuator 144 includes one or more thermostatic devices thatautomatically actuate the spool valve 124 based on temperature. Usingthe thermostatic device eliminates the need for electronic controls,however, the actuator 144 may be electric in other embodiments. Theactuator 144 may include two thermostatic devices 146, 148 that eachcontain wax that automatically expands once an activation temperature isreached. The expanding wax can be used to slide the spool valve 124between different positions. The thermostatic devices 146, 148 may bedisposed within the bore 122 in series as shown. The thermostaticdevices 146, 148 include a first chamber 149 and a second chamber 151,respectively, that are filled with the wax (or suitable material) andinclude an associated piston 150 and 152, respectively. The pistons 150,152 extend into their respective chambers and are acted upon by the wax.The expanding wax drives the pistons 150, 152 to actuate the spool valve124. The thermostatic devices 146, 148 may include return springs (notshown) that de-stroke the pistons or, as illustrated, the resilientmember 142 may de-stroke the pistons 150, 152. The first device 146 maybe fixed within the bore 122 whereas the second device 148 is slidablydisposed within the bore 122. The piston 150 is connected to the seconddevice 148 and the piston 152 is connected to the spool valve 124. Thewaxes within the chambers 149, 151 are configured to activate (expand)at different temperatures. For example, the chamber 149 may activate at20, 30, 40, 50, or 60 degrees Celsius (C) and the chamber 151 mayactivate at 80, 90, or 100 degrees C. These are of course merelyexamples. The activation temperatures can be changed to switch betweenthe modes as desired. The placement of the first and second devices canbe switched so that the device with the lower activation temperature isadjacent to the spool valve 124.

The devices 146, 148 are immersed in the oil 68 to obtain an accuratereading of oil temperature. The housing 120 may include one or moreopenings 154 to place the devices 146, 148 in direct contact with theoil 68. The remaining portions of the valve 74 may also be disposedwithin the oil 68 or may be outside of the oil 68.

The resilient member 142 may bias the spool valve 124 to the firstposition which corresponds to the thermal-management system 62 being inheating mode. In the first position, the lands 128 and 130 seal theirrespective outlets 138 and 140 so that all of the oil entering the inletport 134 is routed to the outlet port 136 between the land 126 and theland 128. The spool valve 124 will remain in the first position until atemperature of the oil 68 reaches an activation temperature (firstthreshold temperature) of the wax within the chamber 149, such as 20degrees C. That is, the thermal-management system is configured to be inheating mode until the oil exceeds the first threshold temperature. Theexpanding wax strokes the piston 150 to the right moving the seconddevice 148 and the spool valve 124 to place valve 124 in the secondposition, which corresponds with the bypass mode. In the secondposition, the piston 150 is stroked, whereas the other piston 152 iscontracted. The spool valve 124 is moved to the third position inresponse to the temperature of the oil 68 exceeding the activationtemperature of the second device 148 (second threshold temperature).That is, the thermal-management system remains in the bypass mode whenthe oil temperature is between the first and second thresholds. When inthe bypass mode, the oil 68 will be heated by friction of thedifferential 40. In the third position, both of the pistons 150 and 152are stoked to place the land 126 over a main passage 160 to route theoil through a secondary passage 162 that is now in fluid communicationwith the outlet 140, which corresponds with the cooling mode.

The above described example is yet one possible embodiment of a passivevalve assembly. In an alternative embodiment, the second device and thepiston may have a hollow center allowing the other piston to extendtherethrough and directly connect to the spool valve 124. Here, thesecond device 148 is stationary within the bore 122. In anotherembodiment, the first and second devices 146, 148 be stacked instead ofbeing arranged in series. Here, each of the devices may be fixed withinthe bore and each piston engages with the spool valve. The pistons havedifferent lengths so that the valve is actuated to different positionswhen the activation temperatures are reached.

Referring to FIG. 6, the heater 92 may be a restriction nozzle 170including a body 172 defining an inlet port 174 that is connected to theconduit 94 and a restriction orifice 176. A diameter of the restrictionorifice 176 is substantially smaller than the diameter of the inlet port174 such that viscous heating occurs. The orifice 176 is sized togenerate sufficient pressure and flow rate of the oil upstream of theorifice 176. The orifice may be sized such that oil pressures of 500 to1000 psi are present upstream of the heater 92.

FIG. 7 shows the viscous-dissipation heater 92 according to anotherembodiment. The heater may be a viscous-dissipation heater assembly 200that, unlike the nozzle 170, is adjustable so that the amount ofrestriction can be increase or decreased based on the oil pressure inconduit 94. If the pump 62 is a positive displacement pump and driven bythe differential 40, the pressure within the system will increase anddecrease as vehicle speed changes. Thus, increase in vehicle speed willincrease the heating of the oil by the viscous-dissipation heaterassembly 200 unless the restriction is reduced. The assembly 200 isconfigured to change the restriction as line pressure changes to producea more uniform heating effect, i.e., reduce restriction as line pressureincreases and increase restriction as line pressure decreases.

The assembly 200 includes a body 202 defining an inlet 204 and an outlet206. A valve 208 is disposed within the body 202 between the inlet 204and the outlet 206. The valve 208 includes a metering portion 210 thatengages with a seat 212 and a driven portion 214 that is external to thebody 202. The valve 208 may be threadably received within the body 202such that rotation of the valve 208 in a first direction moves themetering portion 210 away from the seat 212 to increase the opening ofthe valve (less restriction) and such that rotation of the valve 208 ina second direction moves the metering portion 210 towards the seat 212to reduce the opening (more restriction). Generally, increasing theopening reduces the heating effect and decreasing the opening increasesthe heating effect of the heating valve assembly 200.

An actuator arrangement 216 is configured to rotate the valve 208. Theactuator arrangement 216 may operate based on line pressure so thatelectronics are not required. In other embodiments, however, theactuator arrangement may be electric. The actuator arrangement 216 mayinclude a hydraulic cylinder 218 defining a hydraulic chamber 220. Apiston 222 is disposed within the chamber 220. The piston 222 is biasedin a first direction by a resilient member 224, e.g., a coil spring. Adrive mechanism 226 connects the piston 222 to the driven portion 214 ofthe valve 208. The drive mechanism 226 may be a rack-and-pinionassembly. A rack gear 228 is connected to the piston 222, and a piniongear 230 is connected to the driven portion 214. The pinion 230 includesgear teeth in meshing engagement with gear teeth of the rack 228.Movement of the rack 228 in the first direction (left) rotates the valvein the second direction to increase the opening, and movement of therack 228 in the second direction (right) rotates the valve 208 in thefirst direction to decrease the opening.

The hydraulic chamber 220 is in fluid communication with the conduit 94via a sensing line 232 and an orifice 234 that is defined in the body218 and opens into the hydraulic chamber 220. The sensing line 232 is influid communication with the same conduit as the inlet 204. The oilpressure within the hydraulic chamber 220 is substantially equal to thepressure within the conduit 94. The resilient member 224 biases thevalve 208 to a first position when the line pressure is insufficient tocompress the resilient member 224. The first position has the mostrestrictive opening of the valve to promote sufficient heating at lowerflow rates. When line pressure increases above a threshold, the piston222 will begin to compress the resilient member 224 causing the openingbetween the metering portion 210 and the seat 212 to increase in size.While this reduces the restriction within the assembly 200, theincreased oil flow rate causes the oil to be heated by roughly a sameamount as when in the first position. The actuator arrangement 216 willcontinually adjust the opening of the assembly 200 based on the sensedline pressure to provide a more consistent heating of the oil comparedto a fixed orifice.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes caninclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

What is claimed is:
 1. A differential assembly comprising: a housingdefining an oil sump; a differential disposed in the housing; and athermal-management system including: an oil pump in fluid communicationwith the sump, an oil-to-air heat exchanger external to the housing, anda passive valve assembly having an inlet connected to the pump, a firstoutlet connected to the heat exchanger, and a second outlet connected toa conduit disposed within the housing, the valve assembly further havinga valve movable to a first position in which the inlet and the firstoutlet are in fluid communication and to a second position in which theinlet and the second outlet are in fluid communication, wherein thevalve is configured to be in the first position in response to the oilexceeding a first threshold temperature.
 2. The differential assembly ofclaim 1, wherein the valve is configured to be in the second positionwhen the oil is less than the first threshold temperature.
 3. Thedifferential assembly of claim 1, wherein the valve is configured to bein the second position when the oil is less than the first thresholdtemperature and greater than a second threshold temperature.
 4. Thedifferential assembly of claim 1, wherein the valve assembly further hasa third outlet connected to a viscous-dissipation heater having an inletand an orifice that is smaller than the inlet and sized to heat the oilthrough viscous dissipation, and wherein the valve is further movable toa third position in which the inlet and the third outlet are connectedin fluid communication.
 5. The differential assembly of claim 4, whereinthe valve is configured to be in the third position when the oil is lessthan a second threshold temperature and is configured to be in thesecond position when the oil is between the first and second thresholdtemperatures.
 6. The differential assembly of claim 4, wherein theorifice has a fixed size.
 7. The differential assembly of claim 1,wherein the valve assembly further has a third outlet, and thethermal-management system further includes a viscous-dissipation heaterassembly including: a body defining an inlet connected to the thirdoutlet, a second valve disposed in the body and actuatable to adjust anopening of the viscous-dissipation heater assembly, and an actuatorincluding a hydraulic cylinder in fluid communication with the thirdoutlet, the actuator being driveably connected to the valve andconfigured to actuate the valve to adjust a size of the opening.
 8. Thedifferential assembly of claim 1, wherein the valve is a slidable spool,and wherein the valve assembly further has a passive actuator connectedto the spool and configured to slide the spool between the first andsecond positions.
 9. The differential assembly of claim 8, wherein thepassive actuator includes a thermal expansion material.
 10. Thedifferential assembly of claim 9, wherein the material is wax.
 11. Thedifferential assembly of claim 5, wherein the valve is a slidable spool,and wherein the valve assembly further has a first passive actuator witha first piston configured to slide the spool and a second passiveactuator with a second piston configured to slide the spool, wherein thespool is biased to the third position, the first actuator is configuredto move the spool to the second position in response to the oilexceeding the second threshold temperature, and the second actuator isconfigured to move the spool to the first position in response to theoil exceeding the first threshold temperature.
 12. The differentialassembly of claim 11, wherein the first and second actuators eachinclude a chamber filled with wax configured to actuate the first andsecond pistons, respectively.
 13. The differential assembly of claim 11,wherein the valve defines a bore, and the spool, the first passiveactuator, and the second passive actuator are is disposed in the bore,wherein the first and second passive actuators are arranged in serieswith the first piston connected to the second actuator and the secondpiston connected to the spool.
 14. The differential assembly of claim 1,wherein the pump is driven by the differential.
 15. A differentialassembly comprising: a housing defining an oil sump; a differentialdisposed in the housing; and a thermal-management system including: anoil pump in fluid communication with the sump, a spool valve having aninlet connected to the pump, a first outlet, a second outlet, and aspool slidable to a first position in which the inlet is in fluidcommunication with the first outlet and to a second position in whichthe inlet is in fluid communication with the second outlet, the spoolvalve further having a chamber containing wax configured to move thespool according to a temperature of the wax such that the spool is inthe first position when the temperature of the wax is within a firsttemperature range and is in the second position when the temperature ofthe wax is within a second temperature range.
 16. The differentialassembly of claim 15, wherein the differential includes a carrier, apair of spider gears supported by the carrier, and a pair of side gearsmeshing with the spider gears.
 17. The differential assembly of claim15, wherein the thermal-management system further includes an oil-to-airheat exchanger external to the housing and connected to the firstoutlet.
 18. The differential assembly of claim 15, wherein thethermal-management system further includes a viscous-dissipation heaterdisposed in the housing and connected to the first outlet.
 19. Thedifferential assembly of claim 15, wherein spool valve further has athird outlet, and the spool is slidable to a third position in which theinlet is in fluid communication with the third outlet, the spool valvefurther having a second chamber containing a second wax configured tomove the spool to the third position responsive to the second waxexceeding a threshold temperature.
 20. A viscous-dissipation heaterassembly comprising: a body including an inlet and an outlet; a valveincluding a metering portion disposed in the body between the inlet andthe outlet and a driven portion external to the body, the valve beingactuatable to adjust size of an opening defined between the meteringportion and the body; and an actuator arrangement configured to actuatethe valve, the actuator arrangement including: a hydraulic cylinderdefining a hydraulic chamber and an orifice opening into the chamber andhaving a piston disposed in the hydraulic chamber, wherein the piston isbiased in a first direction and is configured to move in a seconddirection in response to fluid pressure within the chamber overcomingthe bias, and a drive mechanism connected between the piston and thedriven portion, wherein movement of the piston in the first directionreduces the size of the opening and movement of the piston in the seconddirection increases the size of the opening.